ICAO’s Committee on Aviation Environmental Protection proposed using a plane’s maximum take-off weight, or MTOW, to help gauge aircraft emissions and efficiency, the Montreal-based regulatory body said in July. The ICAO council was expected to approve the system this year, it said at the time.

“Standards, not to mention the broad consensus which effective ones require, take time to develop,” Anthony Philbin, a spokesman for ICAO in Montreal, said yesterday by e-mail. “The present timetable looks to it being more fully resolved by end-2014 or possibly sometime in 2015.”

The United Nations-overseen body is crafting its greenhouse gas-reduction plan after the European Union broadened its emissions market to cover airlines in January 2012. Europe can replace its aviation targets with a global measure as long as it’s as stringent as the EU’s, Connie Hedegaard, the bloc’s climate chief, said in February 2012.

Critics of ICAO’s measure include Dimitri Simos, founder of aeronautics software company Lissys Ltd. in Woodhouse Eaves, England, who argues it should include an aircraft’s empty weight rather than the more theoretical MTOW.

“It was flawed at its heart” because it’s an incomplete picture of aircraft emissions, said Simos, whose clients have included Boeing Co. and Airbus SAS. A more stringent metric would stop airlines from “getting away with things that may be commercially beneficial but are bad for the environment” such as using inefficient aircraft, he said.